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Map showing forecast rainfall for December 2023, relative to the December normal.
November 2023 – O Nino, where art thou?
Following on from a September and October which ran quite true to form for our climate drivers (westerly barrages, strong high-pressure anomalies in the Tasman Sea and low-pressure anomalies in the Southern Ocean), you can be forgiven for asking what happened to our strengthening El Nino in November, where pressure patterns across New Zealand didn’t respond to type.
Areas of high pressure were often slow-moving across the Southern Ocean during November with our classic westerly signal only showing up for a brief interlude mid-month. Instead, troughs digging southward from the tropics to the northeast of NZ were a recurring theme, and we have seen much more frequent showery southerly to southeasterly airflows across the country than normal.
This has driven another very wet month for the storm-battered regions of Tairawhiti and Hawke’s Bay; Gisborne Airport saw its 3rd wettest November on record. Wetter than normal conditions have also been observed from the Bay of Plenty to the Wairarapa, and in exposed pockets around Gisborne and southern Canterbury. These areas also saw the coolest temperatures compared to normal last month. Conversely, areas we might typically expect to be rather wet in November such as West Coast South Island have basked in much sunnier, warmer and drier conditions than normal. The majority of the South Island had a drier than normal November.
Climate Drivers – MJO will mix up the weather maps once more in December
Whilst El Nino remains an important player for NZ, the Madden-Julian Oscillation (MJO), a climate driver tracked across the equator, looks set to help shake up the weather maps early in December, much like it did in November. A pulse of the MJO will move from the Maritime Continent (Southeast Asia) into the Western Pacific during the first half of the month. This will lead to more low pressure across the Australasian/NZ regions, with the belt of higher pressure likely displaced further southwards across South Island, similar to November, at least initially.
El Nino is expected to peak in January 2024, continuing through into the autumn months. NZ typically sees the strongest effects of El Nino during late summer/early autumn.
December 2023 Outlook – An unsettled start, but dreaming of a bright Christmas
As noted above, the MJO looks likely to help drive a rather unsettled opening to December, although the South Island might tap into drier weather under that southern ridge more often than not. A couple separate Tasman Sea low pressure systems from the weekend of the 2nd/3rd into the following week look likely to bring rainfall right across the country, but especially across North Island. If the second of these makes a connection with an increasingly active tropics to the north, we may well see some notably heavy falls for northern and northeastern portions of North Island too, but this remains far from certain at this early stage. Keeping a close eye on tropical developments to our north will be important through the first half of December.
Heading beyond mid-month, higher than normal pressures are gradually favoured to return across the Tasman Sea, and to the north of New Zealand, with a more westerly pattern redeveloping across the South Island. This pattern is much more typical of El Nino, as the MJO pulse departs eastward. This will be welcome news for those of us across central and northern NZ who are hoping for some more prolonged sunshine as the summer holidays get started and hay-making season ramps up. These synoptic patterns even offer some early hope of a dry Christmas Day BBQ in these regions!
Rainfall should become more frequent again for western and southern South Island though. With a predominantly westerly flow, eastern sections of South Island may well be in for some very hot early summer days alongside long dry runs too.
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